1. Field of the Invention
The present invention relates to a valve timing control apparatus of an internal combustion engine.
2. Description of Related Art
A previously proposed valve timing control apparatus includes a housing, which is rotated synchronously with a crankshaft, and a vane rotor, which is rotated synchronously with a camshaft. For example, Japanese Unexamined Patent Publication JP2005-325841A (corresponding to U.S. Pat. No. 7,533,695 B2) teaches such a valve timing control apparatus, which changes the rotational phase of the vane rotor relative to the housing toward one of an advancing side and a retarding side by supplying hydraulic fluid into a corresponding one of an advancing chamber and a retarding chamber, which are arranged one after another in a rotational direction and are partitioned by the vane rotor in an inside of the housing. This valve timing control apparatus has a control valve, which controls input and output of the hydraulic fluid relative to the advancing chamber and the retarding chamber.
Specifically, during an operation in a phase change mode (advancing mode or retarding mode) for changing the rotational phase, the control valve feeds the hydraulic fluid, which is supplied from a supply source to a supply port of the control valve, to one of the advancing chamber and the retarding chamber through a feed port (advancing port or retarding port) connected to the supply port. At this time, in a connection passage, which connects the supply port to the feed port, a check valve is operated in response to alternation in an oscillating torque, which is applied from the camshaft to the vane rotor.
First of all, when the oscillating torque is exerted in a direction for increasing a volume of a subject one of the advancing chamber and the retarding chamber, to which the hydraulic fluid is fed from the feed port, a negative pressure is generated in the subject one of the advancing chamber and the retarding chamber. Therefore, in the connection passage, which is connected to the subject one of the advancing chamber and the retarding chamber, the flow of the hydraulic fluid from the supply port to the feed port is enabled by the check valve. Therefore, the hydraulic fluid, which is supplied from the supply source to the supply port, is fed to the subject one of the advancing chamber and the retarding chamber through the feed port, so that the rotational phase of the vane rotor relative to the housing is changed. In contrast, when the oscillating torque is exerted in a direction for reducing the volume of the subject one of the advancing chamber and the retarding chamber, the hydraulic fluid of the subject one of the advancing chamber and the retarding chamber is discharged to the connection passage through the feed port. Thus, in the connection passage, the flow of the hydraulic fluid from the feed port to the supply port is limited by the check valve. Thereby, returning of the rotational phase, which would be caused by the discharge of the hydraulic fluid from the subject one of the advancing chamber and the retarding chamber, is limited.
In JP2005-325841A (corresponding to U.S. Pat. No. 7,533,695 B2), the check valve of the control valve is a spring equipped check valve, in which a valve member is urged by a spring against a valve seat. Therefore, a valve closing speed of the check valve at the time of seating the valve member against the valve seat using a restoring force of the spring is high. However, a valve opening speed of the check valve at the time of lifting the valve member away from the valve seat against the restoring force of the spring is low. Furthermore, the valve member of the check valve of the valve timing control apparatus recited in JP2005-325841A (corresponding to U.S. Pat. No. 7,533,695 B2) is formed as a solid spherical ball. Therefore, in the lifted state of the valve member away from the valve seat, when the hydraulic fluid, which flows toward the feed port in the connection passage, collides against the valve member, a substantial reduction in the amount of pressure loss of the hydraulic fluid may possibly occur. Thereby, the supply of the hydraulic fluid to the subject one of the advancing chamber and the retarding chamber may be delayed, thereby resulting in a reduction in a response speed for adjusting the valve timing, which corresponds to the rotational phase.
Furthermore, Japanese Unexamined Patent Publication JP2009-138611A (corresponding to US2009/0145386A1) teaches another valve timing control apparatus. In this valve timing control apparatus, a sleeve has a supply port, a drain port, an advancing port and a retarding port. The supply port receives the hydraulic fluid from a supply source. The drain port is open to the atmosphere and discharges the hydraulic fluid. The hydraulic fluid is fed to or discharged from the advancing chamber through the advancing port. Also, the hydraulic fluid is fed to or discharged from the retarding chamber through the retarding port. During the operation of the valve timing control apparatus in an advancing mode, which changes the rotational phase to an advancing side, the advancing port and the supply port are communicated with each other to feed the hydraulic fluid to the advancing chamber, and the retarding port is communicated with the drain port to discharge the hydraulic fluid from the retarding chamber. During the operation of the valve timing control apparatus in a retarding mode, which changes the rotational phase to a retarding side, the retarding port and the supply port are communicated with each other to feed the hydraulic fluid to the retarding chamber, and the advancing port is communicated with the drain port to discharge the hydraulic fluid from the advancing chamber.
In the valve timing control apparatus of JP2009-138611A (corresponding to US2009/0145386A1), the drain port, which is formed in the sleeve of the control valve received in the camshaft on the radially inner side of the vane rotor, is opened to the atmosphere through a drain passage that extends through the camshaft. The drain port, which is displaced from the advancing port and the retarding port in the axial direction of the sleeve, is formed such that a circumferential position of the drain port in a circumferential direction of the sleeve coincides with a circumferential position of the drain passage. Therefore, a length of a discharge passage of the hydraulic fluid from the retarding port or the advancing port to the drain passage may possibly become insufficient to cause a reduction in the amount of pressure loss in the discharge passage during the operation in the advancing mode or the retarding mode. In such a case where the amount of the pressure loss at the discharge passage is reduced, i.e., becomes small, an excessive quantity of the hydraulic fluid is discharged from the corresponding one of the advancing chamber and the retarding chamber through the discharge passage. Thereby, a negative pressure is generated in the other one of the advancing chamber and the retarding chamber, to which the hydraulic fluid is currently fed, due to an increase in the volume of the other one of the advancing chamber and the retarding chamber. When the air is drawn into the other one of the advancing chamber and the retarding chamber, an apparent elastic modulus of a mixture of the air and the hydraulic fluid becomes small in the other one of the advancing chamber and the retarding chamber to cause fluctuating movement of the vane rotor. Therefore, it is difficult to achieve a high response speed for adjusting the valve timing, which corresponds to the rotational phase.
Furthermore, in the valve timing control apparatus of JP2009-138611A (corresponding to US2009/0145386A1), an advancing passage extends through the vane rotor and the camshaft to communicate between the advancing chamber and the advancing port, and the advancing port is displaced from the advancing passage in the circumferential direction of the sleeve. Therefore, during the operation in the retarding mode, the amount of pressure loss is increased in the discharge passage, which extends from the advancing passage to the advancing port, so that the response speed for adjusting the valve timing can be improved. However, during the operation in the advancing mode, this discharge passage is used as a feed passage of the hydraulic fluid, which extends from the advancing port to the advancing passage, and the increased amount of pressure loss in this feed passage disadvantageously causes a reduction in the response speed for adjusting the valve timing.